As used herein, certain terms have the meanings ascribed to them.
"Colorant" means a compound added to impart color to a substrate. Colorants can be, for example, pigments or dyes or mixtures of pigments, dyes, etc.
"Spinning efficiency" means the percentage of time that the winder is collecting yarn ("up time") with respect to the total operating time, i.e., (up time/total time).times.100.
"Solution-dyed" means that color is added to a fiber-forming material prior to extruding it into a fiber through a spinneret capillary.
It is known that, unaided, nylon fibers are not sufficiently light resistant for certain applications. For these nylon fibers, exposure to light causes the fiber to discolor and/or to lose physical properties. Light stabilizers have been incorporated in the nylon polymer to address this problem.
There are several classes of light stabilizers. As additives, organic stabilizers (such as hindered phenols) are expensive and have limited effectiveness.
Inorganic salts are generally less expensive and more effective than organic stabilizers but suffer from a different set of problems. For example, copper compounds are inexpensive relative to organic stabilizers but form copper deposits in pipes, spin packs, extruders and other equipment because some of the copper ions reduce to elemental copper during the spinning process. This phenomenon is described in U.S. Pat. No. 3,929,725 to Davis and U.S. Pat. No. 3,947,424 to Tomek. Copper deposits cause spinning difficulties and add significant equipment maintenance expense. Furthermore, the removal of copper deposits generates environmentally undesirable effluent.
A class of stabilizers is known as "hindered amine light stabilizers" or by the acronym "HALS". HALS stabilizer additives may be added to the polymer after polymerization in the form of monomers, low molecular weight additives or incorporated into a polymer backbone. Such stabilizers are typically high cost and low effectiveness compared to copper based stabilizers. In addition, the polymer itself has been modified by polymerizing the amide monomers in the presence of a HALS type of compound. This has been described in WO 15 95/28443.
It is known to solution-dye nylon fibers. For example, solution-dyed nylon fibers are described in U.S. Pat. No. 4,374,641 to Burlone. The problem of light stability is complicated by solution-dyeing. Some colorants actually accelerate the degradation of the host polyamide. Without stabilizing additives, the fiber loses an unacceptable degree of strength during exposure to light or the colorant does not remain "true" to its color. Proper stabilizer selection is made tricky, to say the least. For example, certain pigments are taught to be incompatible with HALS stabilizers. See W. Herbst and K. Hunger, "Industrial Organic Pigments" Verlagsgesellschaft mbH, Federal Republic of Germany, 1993, pp. 467-475
Although WO 95/28443 describes addition of TiO.sub.2 and possibly other colorants, this addition is during the polymerization phase. In the manufacture of a palette of solution-dyed yarns, it is not practical to use polymer that is already colored (not delustered) in polymerization. It should be apparent that the color of the polymer will interfere with the fiber spinner's ability to make a broad spectrum of colors.
In addition, certain colorants or classes of colorants are notoriously difficult to spin and operators experience frustrating breaks during the spinning process. Any method that improves the spinning efficiency of solution-dyed fibers colored with these materials is desirable.
Therefore, there remains a need for an inexpensive method for making a variety of colors in solution-dyed nylon fibers that are stabilized with respect to the polyamide and to the colorant. Such a method should not have any of the disadvantages that are described above.